Capnogram

Graphical representation of the concentration of carbon dioxide in respiratory gases

A capnogram is a graphical representation of the concentration of carbon dioxide (CO₂) in respiratory gases over time. It is an essential tool in anesthesiology, intensive care medicine, and emergency medicine for monitoring a patient's ventilatory status. The capnogram is obtained using a device called a capnograph, which measures the partial pressure of CO₂ in exhaled air.

Components of a Capnogram

A typical capnogram consists of four phases:

Phase I: Baseline

Phase I represents the beginning of exhalation, where the air from the anatomical dead space is exhaled. This air contains little to no CO₂, resulting in a baseline that is close to zero.

Phase II: Expiratory Upstroke

Phase II is characterized by a rapid increase in CO₂ concentration as alveolar gas, which is rich in CO₂, begins to mix with the dead space air. This phase is represented by a steep upward slope on the capnogram.

Phase III: Alveolar Plateau

Phase III, also known as the alveolar plateau, is where the CO₂ concentration reaches a relatively stable level. This phase reflects the exhalation of alveolar gas and is typically a flat or slightly upward-sloping line.

Phase IV: Inspiratory Downstroke

Phase IV marks the beginning of inhalation, where fresh gas is inhaled, and the CO₂ concentration rapidly decreases back to baseline.

Clinical Significance

The capnogram provides valuable information about a patient's respiratory status. Key parameters include:

• End-tidal CO₂ (EtCO₂): The maximum CO₂ concentration at the end of expiration, which is an important indicator of ventilation adequacy.
• Capnographic waveform shape: Changes in the waveform shape can indicate various respiratory or metabolic conditions, such as bronchospasm, hypoventilation, or hyperventilation.

Interpretation

Interpreting a capnogram involves analyzing the waveform shape and EtCO₂ values. Common abnormalities include:

• Elevated EtCO₂: May indicate hypoventilation, increased metabolic rate, or rebreathing of CO₂.
• Decreased EtCO₂: May suggest hyperventilation, decreased cardiac output, or pulmonary embolism.
• Altered waveform shape: A "shark fin" appearance may indicate bronchospasm or airway obstruction.

Applications

Capnography is used in various clinical settings, including:

• Anesthesia monitoring: To ensure adequate ventilation during surgery.
• Cardiopulmonary resuscitation (CPR): To assess the effectiveness of chest compressions and return of spontaneous circulation.
• Sedation: To monitor respiratory status during procedural sedation.

Also see

• Capnography
• End-tidal CO₂
• Respiratory monitoring
• Anesthesia